1. Field of the Invention
The present invention relates to a surface acoustic wave filter and a composite electronic component, which are employed, for example, in a portable telephone.
2. Related Art of the Invention
In recent years, with the development of mobile communication, it has become very desirable to have communication device components higher in performance and smaller in size. The development of balanced semiconductor components such as ICs have advanced in improving antinoise characteristics, and balanced filters for use in an RF stage. Conventionally surface acoustic wave filters have been widely used as RF stage filters in mobile communication devices. Longitudinal-mode surface acoustic wave filters have been capable of balanced/unbalanced conversion owing to the structure of their interdigital transducer electrodes. RF stage filters which use longitudinal-mode surface acoustic wave filters and have balanced input/output terminals are desired to have low-loss, high-attenuation and good balance characteristics.
A conventional longitudinal-mode surface acoustic wave filter having balanced input/output terminals will be described with reference to the drawings.
FIG. 9(a) shows the construction of a conventional longitudinal-mode surface acoustic wave filter having balanced input/output terminals (for example, see Japanese Laid-open No.Hei 6-204781). Referring to FIG. 9(a), the surface acoustic wave filter 901 is constituted by first, second, and third interdigital transducer electrodes (hereinafter referred to as “IDT electrodes”) 902, 903, and 904, and first and second reflector electrodes 905 and 906 on a piezoelectric substrate 911. A group of electrode fingers 902a in two groups of electrode fingers of the first IDT electrode 902 is connected to an unbalanced input/output terminal 909, while the other group of electrode fingers 902b of the first IDT electrode 902 is grounded. A group of electrode fingers 903a in two groups of electrode fingers of the second IDT electrode 903 is connected to a first terminal 907, which is one of balanced input/output terminals, while the other group of electrode fingers 903b is grounded. A group of electrode fingers 904a in two groups of electrode fingers of the third IDT electrode 904 is connected to a second terminal 908, which is the other of the balanced input/output terminals, while the other group of electrode fingers 904b is grounded.
The surface acoustic wave filter having unbalanced and balanced input/output terminals is obtained by being constructed as described above. In actuality, the unbalanced input/output terminal 909 and the balanced input/output terminals 907 and 908 are formed on one piezoelectric substrate on which the IDT electrodes 902 and 903 are also formed. However, the input/output terminals in this example are schematically shown as if they are out of the piezoelectric substrate 911.
Referring to FIG. 9(b), a first inductor 910 for impedance matching in the surface acoustic wave filter 901 is connected between the balanced input/output terminals, i.e., the first and second terminals 907 and 908. By adopting this configuration, impedance matching between the first and second terminals 907 and 908 provided as balanced input/output terminals is achieved.
In a filter such as the above one, to enable this filter to have good balance characteristics, the IDT electrodes 902 to 904 and the first and second reflector electrodes 905 and 906 constituting the surface acoustic wave filter are designed and laid out so that they are closer to a state of being symmetrical about the electrode finger group 902a connected to the unbalanced input/output terminal 909. The entire disclosure of Japanese Laid-Open No.Hei6-204781 is incorporated herein by reference.
FIGS. 10(a), 10(b), and 10(c) are diagrams showing characteristics of the conventional surface acoustic wave filter shown in FIG. 9. However, as an example, the filter which is operated in 188 MHz band is shown. FIG. 10(a) shows a transmission characteristic, FIG. 10(b) shows an amplitude balance characteristic in the passband (from 1805 MHz to 1880 MHz), and FIG. 10(c) shows a phase balance characteristic in the passband. The amplitude balance characteristic is an indication of the amplitude difference between the amplitude of a signal between the first terminal 907 in the balanced input/output terminals and the unbalanced input/output terminal 909, and the amplitude of a signal between the second terminal 908 in the balanced input/output terminals and the unbalanced input/output terminal 909. If the value of this difference is zero, there is no deterioration in amplitude balance characteristic.
The phase balance characteristic is an indication of the shift from 180 degrees of the phase difference between the phase of a signal between the first terminal 907 in the balanced input/output terminals and the unbalanced input/output terminal 909, and the phase of a signal between the second terminal 908 in the balanced input/output terminals and the unbalanced input/output terminal 909. If the value of this difference is zero, there is no deterioration in phase balance characteristic.
The balance characteristics shown in FIGS. 10(b) and 10(c) are characteristics as seen at the first terminal 907 from the second terminal 908. Each of the two balance characteristics seen in the opposite direction is shown by inverting the representation in the graph in FIG. 10(b) or 10(c) about the central horizontal line.
The above-described surface acoustic wave filter, however, has a problem that, despite of the symmetrical construction, the amplitude balance characteristic is −0.3 dB to +1.4 dB in the passband, the phase balance characteristic is −14° to −1° in the passband. Thus there is a large magnitude deterioration in the balance characteristics which are considered one of the important electrical characteristics.
The deterioration in balance characteristics are due not only to the construction but also to coupling between the input/output IDT electrodes by parasitic components.
For example, in the surface acoustic wave filter 901 shown in FIG. 9, the number of electrode fingers in the distance range to the first terminal 907 in the balanced input/output terminals seen from the unbalanced input/output terminal 909 and the number of electrode fingers in the distance range to the second terminal 908 in the balanced input/output terminals seen from the unbalanced input/output terminal 909 are different from each other. Therefore the sums of parasitic components between the corresponding groups of electrode fingers are also different. An unbalance of coupling between the IDT electrodes results therefrom.
In a frequency band, e.g., the 800-900 MHz band, a deterioration in balance characteristics does not occur easily. However, if the system is adapted for use at higher frequencies, transmission and reception of signals at a higher frequency, e.g., 1800 MHz is performed as mentioned above. As the operating frequency is increased, the influence of deterioration in balance characteristics increases and the design must consider this problem.
However, even if the filter is designed, for example, so that (1) the number of electrode fingers in the distance range to the first terminal 907 in the balanced input/output terminals seen from the unbalanced input/output terminal 909 and (2) the number of electrode fingers in the distance range to the second terminal 908 in the balanced input/output terminals seen from the unbalanced input/output terminal 909, are equal to each other. The shapes, however, of the IDT electrodes 903 and 904 cannot be made ideally symmetrical with each other and, therefore, the coupling between the IDT electrodes cannot be balanced.
Further, deterioration in balance characteristics may occur even in balanced filters such as cylindrical filters and dielectric filters supposed to be readily capable of a design of a symmetrical layout as well as in surface acoustic wave filters, and there has been a demand for an effective solution of this problem.